Colour printer characterisation is the process of determining the colour a printer will produce when a certain amount of ink of the various available colours is requested by a given printer. In order to carry out colour printer characterisation, a relationship between signals input to the printer and colorimetric values for printed colours must be established. The relationship is generally expressed as a printer characterisation function. One known method of determining the printer characterisation function for a given printer is by firstly, producing a test page with a predetermined number of sample colour patches, secondly, measuring the colour of each colour patch and finally, interpolating among the measurements. However, this method is time consuming and is not very accurate.
Another known method of determining the printer characterisation function is to determine a printer model, which approximates the function. The main advantage of modelling is that the printer characterisation can be carried out with a comparatively small number of measurements resulting in a large time saving. In determining the printer model, some assumptions are necessary in order to simplify the mathematics involved. The accuracy of the printer model will be limited by these assumptions.
One known printer model is the Neugebauer mixing model. The Neugebauer model is used in modelling and characterising colour printers. The Neugebauer model is used to predict the colour of a print on a given printer, as a weighted average of the XYZ values of the solid overprints of the three primaries (ie. cyan (C), magenta (M) and yellow (Y)). The weights of each colour are determined by the relative dot area coverages of C, M and Y constituting the print. The dot area coverages for the digital input values are determined using a combination of direct measurement and calculation.
As discussed above, the Neugebauer model provides the characterisation function from device values (C,M,Y) to colorimetric values (XYZ). However, for printer characterisation the inverse mapping (ie., from colorimetric values to device values) is required.
Numerical methods are used to invert a printer characterisation function, which is non-linear. However, a problem occurs when a printer characterisation function has more than three inks, as a number of different ink combinations can result in the same colour.
Several methods have been proposed for optimising the Neugebauer model. One such method is discussed in an article by Balasubramanian et al, entitled “Optimisation of the spectral Neugebauer model for printer characterisation”, Journal of Electronic Imaging, April 1999, Vol. 8(2). The method uses weighted spectral regression for optimising the Neugebauer primaries in order to characterise Xerographic printers. However, the method proposed by Balasubramanian et al does not work on ink-jet printers where inaccuracies are found in the optimised Neugebauer model proposed in the article.
It is an object of the present invention to ameliorate one or more of the limitations of the methods described above.